Foundation Settlement Issues

[Bubby]

We did a subsurface investigation of the soils under a slab of a house in which appeared to have heaved or settled. no expansive soils were encountered, nor loose/soft soils encountered. soils below the slab to a depth of 5 feet were observed as natural silty sands (residual soils). Water was encountered under the slab at approximatley 18 inches and was observd to rise to approximatley 6 inches of the slab. No water was observed to rise through the cracks in the slab. Total settlement or heave of 1 to 1.75 inches. I am looking for other options of why this may of happened.

 

[Grouser]

Bubby

For starters: What is the foundation type and depth? Over what time period did the movement occur? Is the area of movement related to the location of sewer/water pipes? Can you get info for nearby similar structures? Has water table fluctuated significantly? What is the frost depth at your site?

A little background data will help the forum members "zero" in on your problem.

 

[Focht3]

[blue]Grouser[/blue] is right - we need more data.

As a general note, have you had the sanitary sewer checked for leaks? Sanitary sewer leaks + shallow GWT + silty or dispersive soils = loss of ground - in some circumstances.

DEFINITELY have the sewer system tested, mapped (really traced, not a plumber's guess) - and completely videotaped. It would cost you $300 to $600 here in Texas; not sure of your local rates and capabilities. But this is a MUST HAVE diagnostic tool. You can't rule the plumbing in - or out - without it.

And get an elevation survey done, referenced to a stable bench mark. Another MUST HAVE diagnostic tool, particularly if litigation is contemplated...



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[GeoPaveTraffic]

I agree completely with the previous posts, also, could you describe the pattern of apparent movement and the crack pattern. Sometimes that leads to a better understanding of what might be going on.

 

[ChrisMcLean]

It does sound like movement due to a leaking drain leaching fines from the soil and causing voids. I've just had one of these and spent £1000 or so on testing before it occurred to me to check the drains....

Unfortunately for the house owner, it later transpired that the drain had been leaking heavily for a number of years under one corner of the house resulting in a large void! I think he ended up costing his insurance company a lot of money.

 

[Focht3]

Chris -

That's a really good case history; any chance you have enough to generate a short technical paper on the subject? It's a problem that's often overlooked; too few really understand and appreciate the damage that can result from this phenomenon. I've been involved in a lot of forensic investigations and have seen voids from plumbing leaks, yet have had other "geotechnical experts" dispute the mechanism as a source of damage to shallow foundations. I'd love to see a good case story published...

Let me know if you'd like a co-author. The Internet is a wonderful facilitator for this kind of collaboration.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[genomty]

Bubby:

As Grouser and Focht3 posted above, additional information will be helpful. For instance you did not mentioned if the encountered water level is the typical water level at your location. In the case that this level is not the level around the area there are some seepage sources to discart. I'll check first, as is also posted above, the pipelines for leaks. If these utilities are not the problem there are other sources and issues that should be reviewed.

Is there an irrigation system surronding the building?

Is the building located at an elevation similar or lower than any underground water storage structure?

Is there water stagnated around the area that may represent a source of seepage?

Is there evidences related to grounwater flows due to the geomorphology around the area?

How is the groundwater level affected by the rainfall season?

 

[Bubby]

more info.
the house is serviced by on-site septic and the system does not have any issues. the crack pattern in the slab indictes heave. Also the beam in the basement has lifted off the beam pocket about 5/8-inch on both ends. The foundation wall only has one crack, and it isnt that serious. a laser level survey of the top of the basement wall indicated that the rear wall was higher (5/8-inch) than the fron wall with a high variability across the rest of the basement wall was observed. i just got the info of when the house was constructed and it was during late fall and early winter of 1999, so now im thinking frost action. i am waiting to hear back on specific daily highs/lows and precip for the time the house was constructed. if you have any other thoughts, shoot them at me. thanks for the help.

 

[Focht3]

... Also the beam in the basement has lifted off the beam pocket about 5/8-inch on both ends. ...

Please describe the foundation system for this house. Is the referenced beam simply supported, or do you mean that there's a 5/8 inch gap beneath the grade beam, and the perimeter beam has lifted?



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[Bubby]

the beam has lifted 5/8-inch from the concrete foundation wall. the beam is a structural steel beam which is supported by the foundation wall and two lally columns. the fiorst floor and framing rest on the beam. The system is a typical residential dwelling. The columns appear to have heaved or the exterior walls appears to have settled. which happened i dont know. there is no as-built survey available. first i thought is was expansive soils, but that isn't the issue. then i thought hydraulic uplift from the water under the slab, but there is no water moving through the cracks in the slab. then the soils under the slab and footings appear to be natural, medium dense, silty sand. good for 3000 psf. now im thinking frost during construction. if anyone has anything else, let me know.

 

[GeoPaveTraffic]

You say the crack pattern indicates heave, are the cracks like X's through the beam column footings? Is there any vertical offset at these cracks? Is there any horizontal offset at these cracks?

When did the cracks first appear?

 

[BigH]

I've seen frost lift a huge slab on grade in weathered Queenston Shale for a computerized warehouse - at time of construction of the slab/thereafter. But, it went down after thaw and no problems reported since. But your's didn't go back down!

 

[Laser28]

I've seen similar problems in foundations where the excavating contractor overzealously dug the excavation, and filled in the over-excavated area with native material. No compaction was applied to the soils, and the result was that one corner of the foundation and wall settled 1/2 inch. In my own house, which is 50 years old, I have a void under one corner and the masonry walls have settled and cracked. That problem is an active spring and I WILL write a paper about that when I dig it up.

 

[Focht3]

Good - post a copy online when you do!



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[genomty]

Bubby:

Perhaps my question sounds obvious for you but, when you refer to on-site septic, are you referring to a underground septic tank?

Do you know? what kind of soils are below 5 feet?

I recently worked in a project where an underground cistern tank was leaking some years and nobody have noticed it. The soils below floor slabs heave and exterior walls settled. The superficial soils down to 4 or 5 feet were clayey sands with a low plasticity and below of them there were plastic and highly plastic clays. When the soils and foundation study was developed this cistern did not exist and was not planned. The guys who perform the study apparently do not consider the effect of the vertical movements related with an eventual water seepage.

 

[andyfabian]

Bubby,

It might be the septic lines have burst and are undermining the foundations; therefore, differential settlement has occurred. If you determine a benchmark that has been established during construction and compare this with the situation at hand, maybe the answer will become apparent.

Interesting situation, let me know what comes of it.

 

[Baldie]

Depending on the geology where you are located, hydro-compactive (aka collapsible) soils could be the problem. Eolian deposits of silt and/or silty sand can be dense based on SPT, but actually contain small voids that collapse upon wetting. Once the soils have been wetted and collapse, they become denser and the voids are no longer evident. Wetting could be occurring from a leaky utility pipe or possibly from poor surface drainage around the home. Footings below the outer walls would settle if the perimeter soils were wetted, but the interior columns would not settle if the soils supporting them remained dry.

I once worked for a firm that had this type of problem come up. The young engineer did not identify the collapse potential in the samples from the site. He also did not check the surficial geologic mapping which clearly indicated eolian deposits at the site and the potential for collapse. Spread footings were recommended, and during construction the contractor was required to moisten and compact the upper one foot of bearing soils. The wetting process induced significant settlement in the footing trench. Fortunately, this was identified early on in construction; otherwise significant damage would likely have occurred. I believe drilled shafts were then designed and installed in a hurry.

 

[Riggly]

I have read all the threads and all seems to make a lot of snse.
However, I hope that I can add something. Is the problem localized only to one particular dwelling house or is there similar problem in any other residential building in the area. I see the general concensus is that this occurence is due to the action of water whether in the liquid or frozen form.
How soon after construction did the problem occur? Is there any evidence of leaking utility lines in the area? I have been in involved in case of severe settlement and cracking of a residential building. However, severe cracking was observed in other houses in the area. A leaking utility was said to be a possible culprit. Research on the land use of the area also indicated that it was used as a dump site several years before and that the foundation was upon several feet of fill. You have mentioned native soil in the foundation, so the latter case is not applicable.

 

[MPENN]

Frost action?? One of my 3R highway projects ran into an area of native american cultural sites. This delayed the work into the late fall/early winter. A 24" drainage outfall line had to be relocated. We secured rights through a commercial driveway (-10% grade)and across the body shops concrete apron to an new outfall. The apron varied in depth from 6" to 18". Our contractor brought in fill to allow for a uniform PCC depth. The new PCC was placed. I got called by the property owner to come and look at some damage. The steel beams had lifted 3/4" at the front exterior wall. Our contractor had placed the PCC neat to the building and frost action heaved the new apron and lifted the CMU columns. I suspect the fill under the PCC was less than suitable.

 

[J1D]

Bubby,
Just noticed this interesting case. Is there outcome from you investigation yet?

The structural system is no quite clear to me from your description: the beam has lifted 5/8-inch from the concrete foundation wall. the beam is a structural steel beam which is supported by the foundation wall and two lally columns. the fiorst floor and framing rest on the beam. Are the lally columns sitting on the slab where the heave occurred (as you doubt)? If the soil is not expansive and no frost under the basement slab, how could the heave happen? In another word settlement of the exterior walls might be the reason?

Regards